Thiophene chemistry and the chemical stability of the thiophene ring hold potential for use of thiophene materials in molecular-based electronics and photonics. In particular, αα′-conjugated thiophene oligomers (nTs) and polymers (polythiophenes-PTs) have attracted great interest as semiconducting elements in organic thin-film transistors (TFTs).[1,2] To be useful in such devices and related structures, the organic material must support a channel of holes or electrons (p- or n-type semiconductor, respectively) created by the gate electrode bias, which switches the device “on”. Furthermore, the charge mobility of the material must be sufficiently large to increase the source-drain on-conduction by many orders of magnitude over the “off” state. The density of the charge carrier in the channel is modulated by voltage applied at the gate electrode.
To date, the most noteworthy examples of this family of compounds are unsubstituted, α,ω- and β,β′-dialkylsubstituted nT (n=4,6), and β-alkylsubstituted PT, where optimized carrier mobilities (0.1–0.6 cm2 V−1 s−1) and on/off ratios (>106) approach those of amorphous silicon.[1e,2a,c,e,3] However, without exception, these systems facilitate hole injection and behave as p-type semiconductors, presumably because the thiophene electron-richness renders negative carriers susceptible to trapping by residual impurities such as oxygen[4]. Even so, increasing the number of thiophene units decreases dramatically environmental (air, light) stability and causes processing and purification difficulties.
Electron transporting (n-type) organic materials are relatively rare. However, developing/understanding new n-type materials would enable applications[5] such as bipolar transistors, p-n junction diodes, and complementary circuits as well as afford better fundamental understanding of charge transport in molecular solids. The major barrier to progress however, is that most n-type organics are either environmentally sensitive, have relatively low field mobilities, lack volatility for efficient film growth, and/or are difficult to synthesize.[5,6,7]
As indicated by the foregoing notations, these and other aspects of and teachings of the prior art can be found in the following:
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[2] (a) W. Li, H. E. Katz, A. J. Lovinger, J. G. Laquindanum, Chem. Mater. 1999, 11, 458. (b) H. E. Katz, J. G. Laquindanum, A. J. Lovinger, Chem. Mater. 1998, 10, 633. (c) J. G. Laquindanum, H. E. Katz, A. J. Lovinger, J. Am. Chem. Soc. 1998, 120, 664. (d) T. Siegrist, C. Kloc, R. A. Laudise, H. E. Katz, R. C. Haddon, Adv. Mater. 1998, 10, 379. (e) H. E. Katz, J. Mater. Chem. 1997, 7, 369. (f) A. Dodalabapur, L. Torsi, H. E. Katz, Science 1995, 268, 270.
[3] (a) H. Sirringhaus, P. J. Brown, R. H. Friend, K. Bechgaard, B. M. W. Lengeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herving, D. M. de Leeuw, Nature 1999, 401, 685. (b) G. Barbarella, M. Zambianchi, L. Antolini, P. Ostoja, P. Maccagnani, A. Bongini, E. A. Marseglia, E. Tedesco, G. Gigli, R. Cingolani, J. Am. Chem. Soc. 1999, 121, 8920. (c) J. H. Shön, C. Kloc, R. A. Laudise, B. Batlogg, Appl. Phys. Lett. 1998, 73, 3574.
[4] Handbook of Heterocyclic Chemistry; A. R. Katritzky Ed.; Pergamon Press: Oxford, 1983.
[5] (a) Y. Y. Lin, A. Dodabalapur, R. Sarpeshkar, Z. Bao, W. Li, K. Baldwin, V. R. Raju, H. E. Katz, Appl. Phys. Lett. 1999, 74, 2714. (b) G. Horowitz, Adv. Mater. 1998, 10, 365. (c) A. Dodalabapur, J. G. Laquindanum, H. E. Katz, Z. Bao, Appl. Phys. Lett 1996, 69, 4227. (d) N. C. Greenham, S. C. Moratti, D. D. C. Bradley, R. H. Friend, Nature 1993, 365, 628. (e) S. Sze, Semiconductor Devices Physics and Technology; Wiley: New York, 1985; p. 481.
[6] (a) C. P. Jarret, K. Pichler, R. Newbould, R. H. Friend, Synth. Met. 1996, 77, 35. (b) R. C. Haddon, J. Am. Chem. Soc. 1996, 118, 3041. (c) G. Horowitz, F. Kouki, P. Spearman, D. Fichou, C. Nogues, X. Pan, F. Garnier, Adv. Mater. 1996, 8, 242. (d) J. G. Laquindanum, H. E. Katz, A. Dodalabapur, A. J. Lovinger, J. Am. Chem. Soc. 1996, 118, 11331.
[7] The transport properties of meta/α,ω-dicyano-6HT/metal structures are highly metal/interface-dependent; TFT carrier signs and mobilities have not been reported: F. Demanze, A. Yassar, D. Fichou, Synthetic Metals 1999, 101 620.